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Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1qeu0Z-003BRC-1Z; Sat, 09 Sep 2023 09:12:05 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 8EBDA300348; Sat, 9 Sep 2023 11:12:04 +0200 (CEST) Date: Sat, 9 Sep 2023 11:12:04 +0200 From: Peter Zijlstra To: Ankur Arora Cc: Linus Torvalds , linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de, rostedt@goodmis.org, tglx@linutronix.de, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED Message-ID: <20230909091204.GB2771@noisy.programming.kicks-ass.net> References: <20230830184958.2333078-1-ankur.a.arora@oracle.com> <20230830184958.2333078-8-ankur.a.arora@oracle.com> <20230908070258.GA19320@noisy.programming.kicks-ass.net> <87v8cj9alq.fsf@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87v8cj9alq.fsf@oracle.com> X-Rspamd-Queue-Id: 87F14180010 X-Rspam-User: X-Stat-Signature: 15xb1srjwmcajw1p6d9ippnon6te4tri X-Rspamd-Server: rspam01 X-HE-Tag: 1694250740-440281 X-HE-Meta: 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 waAw1LtX AfDfw9q7RMNzzbeO+bT/uMEfGukTCsikKNI0DvvLBwJ1qeNZDrEf0+vWsmWkZML7ugwEEXuJMXo1X6luie8LzvcSi8M5MAu3DPcCxjhNVNlBXFEGQD3pckJ5h/othdvr3GDi7zKVWgjD9+iNABJCmLUxvx+fUZuMLPB9zeIPVSjaLwNLrl4JjmINs4Q== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Sep 08, 2023 at 10:30:57PM -0700, Ankur Arora wrote: > > which actually makes me worry about the nested irq case, because this > > would *not* be ok: > > > > allow_resched(); > > -> irq happens > > -> *nested* irq happens > > <- nested irq return (andapreemption) > > > > ie the allow_resched() needs to still honor the irq count, and a > > nested irq return obviously must not cause any preemption. > > IIUC, this should be equivalent to: > > 01 allow_resched(); > 02 -> irq happens > 03 preempt_count_add(HARDIRQ_OFFSET); > 04 -> nested irq happens > 05 preempt_count_add(HARDIRQ_OFFSET); > 06 > 07 preempt_count_sub(HARDIRQ_OFFSET); > 08 <- nested irq return > 09 preempt_count_sub(HARDIRQ_OFFSET); > > So, even if there were nested interrrupts, then the !preempt_count() > check in raw_irqentry_exit_cond_resched() should ensure that no > preemption happens until after line 09. Yes, this.